Selective adsorption of Pb 2+ from acid mine drainage through a poly (hydroxamic acid) (PHA) ligand: an HSAB-guided coordination mechanism

1 Introduction

Acid mine drainage (AMD) represents a formidable global environmental challenge, primarily precipitated by extensive mining activities and the subsequent oxidation of sulfide minerals (Esmaeili et al., 2019). AMD is a complex and long-lasting phenomenon that forms slowly through physical and chemical reactions initiated by rainfall and microbial activity and can persist for many years following the closure of a mine (Levio-Raiman et al., 2021). AMD is defined as acidic water produced when sulfide-bearing minerals, such as pyrite (FeS₂), are exposed to oxygen and water. Pyrite, the most common sulfide mineral on Earth, often occurs alongside other metal-sulfide minerals, which contribute to acid formation through the following chemical reactions (1-6) (Ódri et al., 2020; Menzel et al., 2021; Xia et al., 2023):

2 F e S 2 s + 7 O 2 + 2 H 2 O ↔ 2 F e 2 + + 4 S O 4 2 − + 4 H + (1)

4 F e 2 + O 2 + 4 H + ↔ F e 3 + + 2 H 2 O (2)

F e 3 + + 3 H 2 O ↔ F e O H 3 + 3 H + (3)

F e S 2 s + 14 F e 3 + + 8 H 2 O ↔ 15 F e 2 + + 2 S O 4 2 − + 16 H + (4)

Similarly, leaching dissolves metal-sulfide minerals associated with pyrite through subsequent chemical reactions (Menzel et al., 2021):

C u F e S 2 s + 4 O 2 ↔ C u 2 + + 2 S O 4 2 − + F e 2 + (5)

C u F e S 2 + 4 F e 3 + ↔ C u 2 + 2 S 0 + 5 F e 2 + (6)

P b S + 2 O 2 ↔ P b 2 + + S O 4 2 − (7)

Z n S + 2 O 2 ↔ Z n 2 + + S O 4 2 − (8)

As a result, bioleaching activity generates various contaminants and ions, including metals and sulphates, which can pollute the environment surrounding mining operations. This exposure accelerates the oxidation of minerals, producing sulfuric acid that dissolves and mobilises heavy metals into nearby water systems (Akcil and Koldas, 2006). Typical physicochemical attributes of AMD include pronounced acidity (low pH), substantial sulfate loadings, and a diverse array of divalent heavy metal species, including Cu²⁺, Ni²⁺, Zn²⁺, Cd²⁺, and Pb²⁺ (Esmaeili et al., 2019; Mzinyane, 2022). Untreated AMD effluents represent a grave environmental threat; the hazardous metallic species within are non-biodegradable and possess the potential to disseminate through hydrological systems, contaminating critical water resources. This contamination disrupts ecological balance and leads to a range of severe environmental consequences.

Furthermore, these heavy metals can bioaccumulate in living organisms, adversely affecting both human health and natural ecosystems. While AMD typically comprises a heterogeneous matrix of metallic species, the current investigation prioritizes Pb²⁺ as a target pollutant owing to its acute toxicity, environmental recalcitrance, and bio-accumulative potential. The ingestion or exposure to Pb⁺² results in the dysregulation of essential metabolic functions in humans, fauna, and flora, while simultaneously manifesting pronounced cytotoxic, mutagenic, and carcinogenic properties. Therefore, its targeted removal is critical for mitigating environmental and health risks associated with contaminated water systems. Considering these deleterious health impacts, the World Health Organization (WHO) and the Bureau of Indian Standards (BIS; IS 3025) have instituted a rigorous permissible threshold of 0.01 mg/L for lead in potable water (Kumar et al., 2022). The degradation of freshwater reservoirs by AMD represents a critical threat to the long-term viability of the global mining industry and the health security of human populations across the globe. This issue is particularly critical in regions that already face limited potable water supplies (Mzinyane, 2022).

Traditional AMD treatment methods, such as active and passive systems, are the two broad categories into which AMD therapy technologies fall. The quality of AMD is enhanced by passive treatments, which need no upkeep and rely on naturally occurring geochemical and biological processes (Gazea et al. (1996). Despite being inexpensive and simple to maintain, these techniques may not be as effective over the long term due to issues such as surface passivation caused by metal precipitate formation, reduced ability to reduce sulphate concentrations, and possible localised environmental effects (Menzel et al., 2021).

Active treatments employ a variety of engineering unit operations to modify the physicochemical properties of AMD. Chemical neutralisation processes are the most utilised of these in the industrial setting.

Lime addition is used in the highly popular high-density sludge (HDS) process to increase pH levels and facilitate the remediation of heavy metals and sulphates from mine effluent through the following reactions: where M symbolizes a divalent metallic cation. AMD mitigation triggers the generation of solid precipitates, such as gypsum and metal hydroxides, via neutralization-induced precipitation, as characterized by reactions (9) and (10). The physicochemical attributes and mineralogical composition of these precipitates are governed by determinants such as the specific cationic species profile, ambient pH, and their respective initial concentrations within the AMD matrix. Although this method effectively reduces metal concentrations, it also has notable drawbacks, including elevated sulfate levels in the effluent and the production of large quantities of gypsum and metal hydroxide waste. Managing this waste requires secure disposal to prevent metal re-dissolution, and the process offers limited potential for metal recovery as valuable by-products (Menzel et al., 2021; Chen et al., 2021).

2 H + + S O 4 2 − + C a O H 2 ↔ C a S O 4 · 2 H 2 O (9)

M 2 + + S O 4 2 − + C a O H 2 + 2 H 2 O ↔ M O H 2 + C a S O 4 · 2 H 2 O (10)

Conventional techniques for treating acid mine drainage (AMD), such as membrane filtration, ion exchange, and chemical precipitation, are often costly, generate secondary waste, and exhibit limited selectivity toward specific contaminants. While these methods can reduce metal concentrations, their high operational costs and challenges with waste management limit their sustainability (Masindi et al., 2022). In contrast, adsorption onto natural materials offers a more environmentally friendly and cost-effective alternative. Natural adsorbents, such as pine cones or agricultural wastes, can be chemically modified with chelating agents, such as poly (hydroxamic acid), which introduce additional carbonyl and hydroxyl groups, thereby enhancing their capacity for strong metal-ion binding and improving removal efficiency.

Despite significant progress in adsorption-based treatment of acid mine drainage (AMD), most existing studies remain limited to single-metal systems under controlled laboratory conditions, with insufficient attention given to competitive adsorption behaviour in chemically complex, multi-ion environments (Torres-Castañón et al., 2025). In particular, the selective removal of Pb²⁺ from AMD remains poorly understood, as co-existing metal ions compete for active binding sites, often reducing process efficiency and limiting practical applicability.

Moreover, while adsorption mechanisms are frequently inferred from kinetic and isotherm models, these approaches alone do not adequately capture the underlying coordination chemistry governing metal–ligand interactions. As a result, there remains a critical gap in linking adsorption performance to fundamental chemical principles that can explain and predict selectivity in multi-metal systems.

Addressing these limitations, the present study makes three distinct contributions. First, it demonstrates selective Pb²⁺ adsorption within a real multi-metal AMD matrix, moving beyond conventional single-solute systems to reflect realistic environmental conditions. Second, it introduces a Hard–Soft acid–Base (HSAB)-guided framework to interpret and rationalise metal selectivity, providing a theoretically grounded explanation of adsorption behaviour based on donor–acceptor compatibility. Third, it establishes a mechanistic pathway dominated by inner-sphere chelation via hydroxamate functional groups, supported by kinetic, isotherm, and thermodynamic evidence, thereby advancing understanding beyond empirical model fitting. In doing so, this study positions hydroxamate-functionalised biomass as a selective, mechanism-driven adsorbent system capable of targeted Pb²⁺ recovery from complex AMD environments, with implications for both wastewater remediation and resource recovery.

To the best of our knowledge, this work is one of the first to thoroughly explain Pb²⁺ selectivity on hydroxamate-functionalized biomass by simultaneously integrating Hard–Soft Acid–Base (HSAB) theory, competitive multi-metal adsorption, and validation in actual AMD systems.

2 Materials and methods

2.1 Adsorbent preparation

The adsorbent was created in accordance with the research of Mzinyane (2022), Mzinyane et al. (2021), and its characteristics were confirmed using SEM, TGA, FTIR, and XRD. These analyses verified that the hydroxamate-functionalized material’s morphological, thermal, and chemical characteristics continued to match previously published data.

2.2 Reagents and solutions

All reagents, including H₂O₂ (30%), ammonium ceric nitrate (98.5%), Fe(OH)₃ (99%), HNO₃ (70%), acrylic acid (99%), HCl (32%), acetone (99.5%), tetrahydrofuran (99.5%), diethyl ether (99%), ethyl chloroformate (99%), N-methylmorpholine (99%), hydroxylamine hydrochloride (99%), acetic acid, and sodium nitrate, were purchased from Merck and Sigma-Aldrich (South Africa) and used without further purification.

A primary stock solution of Pb²⁺ (1000 mg/L) was prepared by dissolving analytical-grade lead sulfate (PbSO₄) in deionised water, which was then used to prepare working solutions of various concentrations. The pH of the solutions was adjusted using 0.1 M HCl or 0.1 M NaOH.

2.3 Batch adsorption experiments

Equilibrium adsorption studies were carried out using 200 mL polypropylene bottles. Aliquots of 100 mL (Pb²⁺ solution) were treated with varying masses of the PHA adsorbent to evaluate the effect of adsorbent dosage. The adsorbent-solution mixtures were mechanically agitated at 200 rpm on an orbital shaker for the duration of the contact time. The residual Pb²⁺ concentration was measured via Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). The equilibrium adsorption capacity (q_e) and the percentage removal (%R) were calculated using the following Equations 1, 2 (Allaoui et al., 2024): where C₀ and C_e are the initial and equilibrium concentrations (mg/L), V is the solution volume (L), and M is the adsorbent mass (g).

q e = C 0 − C e V M × 100 (11)

% R = C 0 − C e C 0 × 100 (12)

2.3.1 Effect of pH

The influence of pH was evaluated by adjusting the pH of 50 mg/L Pb²⁺solutions to 2, 4, 6, 8, 10, and 12 before adding 0.05 g of adsorbent. The mixtures were agitated at 200 rpm for 2 h at room temperature. Subsequently, the adsorbent was separated from the solution by filtration.

2.3.2 Effect of agitation time

To determine the effect of agitation time, 0.05 g of adsorbent was added to 100 mL of a 50 mg/L Pb²⁺ solution. The mixture was maintained at a pH of 6, agitated at 200 rpm, and kept at room temperature. Aliquots were taken at 30-min intervals over 240 min to measure the residual Pb²⁺ concentration. Afterwards, the adsorbent was separated from the solution by filtration.

2.3.3 Effect of adsorbent dosage

To determine the optimal adsorbent dosage, masses ranging from 0.05 to 1.50 g were added to 100 mL of a 50 mg/L Pb²⁺ solution. The mixture was maintained at pH 6, agitated at 200 rpm for 2 h, and kept at room temperature. The adsorbent was then separated from the solution by filtration.

2.3.4 Effect of initial concentration and temperature

To investigate the influence of initial concentration and temperature on adsorption, a constant amount of adsorbent (1.0 g) was added to 100 mL of lead (II) (Pb²⁺) solution. The initial Pb²⁺ concentrations ranged from 25, 50, 75, 100, 125, and 150 mg/L. The solutions were agitated at 200 rpm for 2 h, with the temperature systematically varied from 26 °C to 41 °C, and the adsorbent was then separated from the solution by filtration.

2.4 Adsorption kinetics

The adsorption kinetics of Pb²⁺ were studied by introducing 1 g of the adsorbent into a 200 mL glass flask containing a 100 mg/L Pb²⁺ solution. The flasks were shaken at 200 rpm in a reciprocating shaker at 26 °C. Triplicate samples were withdrawn at various time points (0, 0.25, 0.75, 1, 1.5, 2, 3, 4, 8, 12, and 24 h), centrifuged at 6,000 rpm for 15 min, and filtered through a 0.45 µm filter paper. The residual Pb²⁺ concentration was measured via ICP-OES. The adsorption capacity of metal ions is represented by q_e at equilibrium and q_t at a given time t (minutes), both expressed in milligrams per Gram (mg/g). The adsorption kinetics were evaluated using the pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models. The rate constants for the PFO and PSO models are denoted as k₁ (min^-1) and k₂ (g/mg/min), respectively. The linearised forms of the kinetic models are presented in Equations 3, 4 (Dal and Onursal, 2023):

Pseudo − first − order PFO model : ln q e − q t = ln ⁡ q e − k 1 t (13)

Pseudo − second − order PSO model : t q t = 1 k 2 q e 2 + 1 q e t (14)

In these equations, q_e (mg/g) and q_t (mg/g) represent the amounts of adsorbate adsorbed at equilibrium and at time t (min), respectively.

2.5 Equilibrium adsorption isotherms

The method used to assess the equilibrium adsorption behaviour of Pb²⁺ was in line with the kinetic experiments. To ensure adsorption equilibrium was reached, a 12-h contact time was used. To ensure reproducibility, every experiment was performed in triplicate. To clarify the adsorption mechanism, the equilibrium data were fitted to the Langmuir, the Freundlich, and the Temkin adsorption isotherm models. Equations 5–7 display the linearised versions of the corresponding isotherm equations (Tanko et al., 2026):

Langmuir isotherm : C e / q e = 1 / b q max + C e / q max (15)

Freundlich isotherm : log ⁡ q e = log ⁡ K F + 1 n log ⁡ C e (16)

Temkin isotherm : q e = R T b T ln K + R T b T ln ⁡ C e (17)

Equation 7 relates the Temkin constant b to the heat of adsorption parameter. In this model, K_T (L/g) is the Temkin equilibrium binding constant, T(K) is the absolute temperature, and β (J/mol) is the constant associated with the heat of sorption (Abin-Bazaine et al., 2022). The relationship is given by Equation 18:

b = R T β (18)

Equation 9 was used to determine the dimensionless separation factor (R_L), further to assess the favourability of adsorption within the Langmuir framework (Tanko et al., 2026):

R L = 1 1 + K F C e (19)

The equilibrium adsorption behaviour was evaluated using the Langmuir, Freundlich, and Temkin isotherm models, where q _*e* (mg/g) represents the adsorption capacity at equilibrium and C _*e* (mg/L) denotes the equilibrium concentration of the adsorbate. The Langmuir model describes monolayer adsorption onto a homogeneous surface with a maximum capacity of q _max (mg/g) and an affinity constant K _*L* (L/mg); the nature of this process is further characterised by the dimensionless separation factor R _*L*, where values of R _*L* > 1, R _*L* = 1, and 0<R _*L* < 1 indicate unfavourable, linear, and favourable adsorption, respectively. Conversely, the empirical Freundlich model accounts for adsorption on heterogeneous surfaces, with the constants K _*F* ((mg/g) (L/mg)1/n) and n representing adsorption capacity and intensity, respectively, as derived from the linear regression of logq _*e* versus logC _*e*. Finally, the Temkin isotherm incorporates the effects of indirect adsorbate-adsorbate interactions, where K _*T* (L/g) is the equilibrium binding constant and b _*T* (J/mol) relates to the heat of adsorption, utilising the universal gas constant R (8.314 J/mol·K) and absolute temperature T (K) to describe the linear decrease in adsorption heat with surface coverage.

2.6 Adsorption equilibrium and thermodynamics

The adsorption capacity and thermodynamic parameters were investigated by contacting 1.0 g of adsorbent with 100 mL of Pb²⁺ solutions at initial concentrations of 25, 50, 75, 100, 125, and 150 mg/L. The experiments were conducted at pH six for 2 h at different temperatures (299, 304, 309, and 314 K). Thermodynamic analysis was performed to determine whether the adsorption of Pb²⁺ onto the PHA ligand proceeded via physisorption or chemisorption. Physisorption involves weak van der Waals interactions and is generally reversible, whereas chemisorption involves stronger chemical bond formation between the adsorbent and metal ions and is typically irreversible. The feasibility and nature of adsorption were evaluated using thermodynamic parameters, namely, Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°). The Gibbs free energy change was calculated using Equation 20 (Mzinyane et al., 2023):where R is the universal gas constant (8.314 J/mol/K), T is the absolute temperature (K), and K is the distribution coefficient defined in Equation 21 (Mzinyane et al., 2023) as:

Δ G ° = − R T ⁡ ln ⁡ K (20)

K = q e C e (21)

In Equation 21, where q_e (mg/g) is the equilibrium adsorption capacity and C_e (mg/L) is the equilibrium concentration of Pb²⁺ in solution. A negative value of ΔG° indicates that the adsorption process is spontaneous. The relationship between Gibbs free energy, enthalpy, and entropy is given in Equation 22 by:

Δ G ° = Δ H ° − T Δ S (22)

Combining Equations 20, 22 yields Equation 23:

l n K = − Δ H ° R T + Δ S ° R (23)

2.7 Sampling and preparation of AMD samples for Pb²⁺ removal

The sampling procedure was adapted from Tutu et al. (2008), Mzinyane (2022), further modified for the present Pb(II) study. Polypropylene (PP) bottles that had been pre-cleaned with 10% nitric acid (HNO₃) for 12 h were used to collect acid mine drainage (AMD) from a chrome mine in Witbank, Mpumalanga Province, South Africa (Figure 1a map of South Africa and (b)satellite image). Four samples were collected from each location, primarily from ponds surrounding the mining area. Table 1, together with the geological location depicted in Figure 1, presents the in situ geochemical characteristics measured at each sampling site, including temperature (°C), pH, electrical conductivity (EC, mS/cm), and redox potential (Eh, mV), which were recorded using a portable multiparameter water quality meter (Palintest test kit). Samples were transported to the laboratory under cooled conditions (4 °C), filtered, and acidified with 3 M HNO₃ for cation analysis. Pb²⁺ concentrations in the AMD were determined using inductively coupled plasma optical emission spectroscopy (ICP-OES). Working solutions (100 mg/L Pb²⁺) were prepared by diluting the AMD with deionised water. The pH of these solutions was adjusted to 6.0 using 0.1 M HCl or NaOH before adsorption experiments.

TABLE 1. Field parameters and chemical composition of surface water samples.

3 Results and discussion

3.1 Batch studies

3.1.1 Effect of time

The impact of solution agitation time on Pb²⁺ removal with a PHA ligand was shown in Figure 2a. The first phase corresponded to rapid, strong adsorption to 77 mg/g, made possible by the material’s large number of empty sites. Furthermore, the concentration gradient was greater, and the PHA ligand’s active sites were more readily accessible during the early phases of adsorption (Ozcan et al., 2024). However, the adsorption capacity then slowed down, which may have been due to saturation. A subsequent decrease might have occurred as the adsorbent’s pores became clogged by the adsorbed Pb²⁺ ions or by reaction by-products. Consequently, pore blockage restricted the diffusion of Pb²⁺ ions into the internal binding sites, which resulted in decreased removal efficiency at prolonged contact times (Somyanonthanakun et al., 2023). Furthermore, the adsorbent’s pore shape, surface area, and chemical composition all significantly affect how quickly and efficiently it becomes saturated and clogged. Research by Somyanonthanakun et al. (2023) demonstrates that Pb²⁺ uptake on modified activated carbon is not exclusively dictated by physical attributes like surface area and porosity. Instead, the adsorption process is significantly influenced by the presence of surface functional groups, though prolonged contact can lead to pore clogging that impedes ion diffusion to internal binding sites.

3.1.2 Effect of adsorbent dosage

Figure 2b illustrates the influence of adsorbent dosage on Pb²⁺ adsorption. The adsorption capacity increased from about 69 mg/g to 86 mg/g as the dosage increased up to 1.0 g, after which a slight decrease was observed. The initial increase was due to the higher number of active binding sites available for Pb²⁺ ion capture (AlNeyadi, 2025). However, beyond 1.0 g, the adsorption capacity per unit mass of adsorbent decreased due to particle aggregation, leading to overlapping adsorption sites and a reduction in effective surface area (El-Nazer et al., 2025). The optimal dosage was therefore found to be 1.0 g, balancing surface area and site accessibility.

3.1.3 Effect of pH

The results in Figure 2c showed a strong pH dependence of Pb²⁺ adsorption, with adsorption capacity increasing from approximately 64 mg/g at pH two to about 73 mg/g at pH 6, and remaining nearly constant thereafter. At low pH values, excess hydrogen ions competed with Pb²⁺ ions for available adsorption sites on the hydroxamic acid functional groups, thereby reducing Pb²⁺ uptake. As pH increased, deprotonation of the hydroxamic acid groups enhanced their ability to chelate Pb²⁺ ions via the oxygen donor atoms. At pH values exceeding 6.0, Pb²⁺ uptake attained an equilibrium plateau. This stabilization is potentially ascribed to the precipitation of insoluble Pb(OH)₂ species or the attainment of electrostatic equilibrium on the adsorbent surface. The maximum adsorption capacity for Pb²⁺ was achieved at an optimum pH of 6. According to Citarella et al. (2021), the strong metal-chelating ability of hydroxamic acids (HAs) makes them effective metal-binding groups, particularly for Fe(III) and Zn(II), and HA derivatives have been widely applied as metalloenzyme inhibitors. This report supported the current findings, as the hydroxamic acid functional groups appeared to have effectively coordinated Pb²⁺ ions, thereby enhancing adsorption via complexation and chelation.

3.1.4 Effect of initial concentration and temperature

Figure 2d demonstrates the way temperature and Pb²⁺ concentration affect adsorption capacity. The adsorption of Pb²⁺ ions was observed to have been dependent on temperature, as illustrated in Figure 2d. Both the starting Pb²⁺ concentration and the temperature increase from 299 to 314 K resulted in a consistent improvement in the adsorption capacity. This trend indicated that the adsorption process was endothermic. The greater mobility of Pb²⁺ ions, which improved their diffusion across the boundary layer and into the active sites of the PHA ligand, was indicated by the rise in adsorption capacity at higher temperatures. Higher temperatures also appeared to activate more adsorption sites by increasing the flexibility of the polymeric matrix and promoting stronger coordination interactions between Pb²⁺ ions and oxygen donor atoms within the hydroxamic acid functional groups (Budnyak et al., 2020; Zhang et al., 2025). Furthermore, the decrease in solution viscosity at higher temperatures likely improved mass transfer between the solution and the adsorbent surface. These observations collectively supported the conclusion that Pb²⁺ adsorption onto the PHA ligand was temperature-dependent and predominantly governed by an endothermic chemisorption mechanism.

3.2 Proposed adsorption mechanism

The adsorption of Pb²⁺ onto the PHA ligand proceeds through a sequential pathway involving interfacial hydration, electrostatic enrichment, ligand activation, inner-sphere chelation, and secondary coordination interactions, as illustrated in Schematic 1. Pb²⁺ adsorption onto the poly (hydroxamic acid) (PHA) ligand is controlled by ligand-metal coordination interactions, which go beyond traditional adsorption models and are better understood in the context of coordination chemistry.

Upon contact with aqueous solution, the modified matrix hydrated, forming a structured interfacial water layer around hydroxyl, carboxylate, and hydroxamic acid groups, thereby enhancing ion diffusion toward reactive domains. The rapid initial uptake observed in the kinetic profile (Section 3.1.1) showed that adsorption increased sharply to 77 mg/g during the early contact period, confirming the presence of abundant accessible surface sites and fast external mass transfer (Cheng et al., 2017). Hydrated Pb²⁺ species were attracted to the negatively charged sites created by the pH-mediated deprotonation of surface functional groups during the adsorption process. Commonly used bidentate ligands called hydroxamate functional groups (–CONHO^-) coordinate metal ions through nearby carbonyl (C=O) and deprotonated hydroxyl (N–O^-) oxygen atoms to create stable five-membered chelate rings (Codd, 2008; Citarella et al., 2021). Since of its high polarizability and borderline-soft Lewis acid nature, this coordination mode is especially advantageous for Pb²⁺ (Pearson, 1963; Wang et al., 2020).

During formal coordination, these ions first formed outer-sphere surface complexes. The strong pH sensitivity seen in Section 3.1.3, where a decrease in proton competition and an increase in surface charge density caused an increase in adsorption capacity from 64 mg/g (pH 2) to 73 mg/g (pH 6), supports this theory (Zhu et al., 2017). Under these near-neutral conditions, hydroxamic acid groups underwent deprotonation to generate hydroxamate anions with enhanced donor strength, thereby activating the ligand for metal binding and explaining the optimal adsorption observed at pH 6 (Section 3.1.3). Due to its stereochemically active 6s² lone pair, Pb²⁺ usually displays hemidirected geometries and varied coordination numbers (4–6) from the standpoint of coordination chemistry. Asymmetric coordination environments are produced as a result, allowing for various coordination modes on the PHA surface and flexible ligand binding (Batka and Farkas, 2006; He et al., 2021).

To promote the formation of inner-sphere complexes with hydroxamate ligands, Pb²⁺ partially dissolves after the activation phase. Pb²⁺ is mostly bound by mononuclear bidentate chelation at low surface coverage, in which a single hydroxamate group uses O,O-coordination to produce a stable inner-sphere complex. This interaction is the main mechanism for immobilization and results in the development of five-membered chelate rings. Stable five-membered chelate rings are produced through O,O-bidentate coordination, which involves the carbonyl oxygen and the deprotonated hydroxyl oxygen (N–O^-). A single Pb²⁺ ion is coordinated by several hydroxamate ligands or nearby functional groups in multidentate and bridging configurations, which develop as surface coverage rises. These surface-bridged or multinuclear complexes are compatible with Langmuir-type monolayer adsorption on energetically equivalent locations and show cooperative binding interactions (Codd, 2008; Rahman et al., 2021).

Lead immobilization is mostly caused by this chelation process (Codd, 2008). Direct evidence for this coordination mode was provided by the excellent fit of the pseudo-second-order kinetic model (R² = 1) and the accurate prediction of qe at 85 mg/g (Section 3.5), demonstrating that adsorption was governed primarily by surface complexation rather than diffusion-controlled physisorption. At higher surface coverage, adjacent hydroxamate ligands coordinated simultaneously with a single Pb²⁺ ion, forming bridging or multinuclear surface complexes consistent with the Langmuir isotherm behaviour reported in Section 3.4 (R² = 0.999), which indicated adsorption onto a finite number of energetically equivalent high-affinity sites. Although hydroxamate chelation represented the principal uptake mechanism, additional binding arose from carboxylate and hydroxyl groups. These auxiliary functional groups improve overall adsorption capacity through reduced secondary coordination and ion-exchange interactions, but they do not control selectivity, which is still primarily determined by hydroxamate-driven chelation. Consequently, the total adsorption capacity is increased without the major chelation pathway being replaced (Sections 3.1.2, 3.4) (Codd, 2008).

Furthermore, Hard–Soft Acid–Base (HSAB) theory can explain the preferential adsorption of Pb²⁺ in the presence of competing metal ions. Pb²⁺ has a considerable attraction for oxygen donor ligands with intermediate polarizability, including hydroxamate groups, while being a borderline-soft acid. Conversely, there is less effective competition between competing hard acids (like Mn²⁺) and hydrolyzed species (like Fe³⁺ under circumneutral circumstances) (Pearson, 1963; Kong et al., 2020). Additionally, ligand exchange processes that favour the production of more stable inner-sphere hydroxamate complexes are made possible by Pb²⁺ speciation in sulfate-rich AMD environments, such as PbSO₄ complexes (He et al., 2021).

All of these results point to a coordination-driven adsorption mechanism where the removal of Pb²⁺ is controlled by (i) donor–acceptor compatibility directed by HSAB, (ii) bidentate and multidentate hydroxamate chelation, and (iii) flexible coordination geometry made possible by the stereochemical active lone pair of Pb²⁺. This interpretation provides a more reliable and predictive framework for selective metal removal in complicated aqueous systems by connecting macroscopic adsorption behaviour to molecular-scale coordination chemistry.

3.3 Comparative initial adsorption behaviour in single-metal and AMD systems: An HSAB perspective

Figure 3 shows the temporal evolution of adsorption capacity (q_e) and Pb²⁺ removal efficiency in both single-metal and acid mine drainage (AMD) systems. Within the first 10–20 min, both matrices experienced a fast-initial uptake phase. After around 60 min, there was a slow movement toward equilibrium. This kinetic profile indicates that the availability of surface-active sites was the main factor influencing the early stage of adsorption. At the same time, the later stage was controlled by site saturation and intraparticle diffusion.

According to the Hard–Soft Acid–Base (HSAB) theory, Pb²⁺ is classified as a borderline soft acid. It therefore exhibits a strong affinity for soft and borderline donor atoms, particularly oxygen- and nitrogen-containing functional groups (Wang et al., 2020; Datta, 1992; Pearson, 1963; Pustam and Spiro, 2010). The PHA ligand possesses abundant–C=O, –N–OH, and–OH functional groups, which serve as effective coordination sites for Pb²⁺. Rapid inner-sphere complexation and chelation, which resulted in the creation of stable surface complexes, were responsible for the sharp initial increase in qe and removal efficiency (Schematic 1).

Notably, the AMD matrix exhibited marginally higher adsorption capacity and removal efficiency than the single-metal system. This behaviour suggests that the presence of multivalent cations and elevated ionic strength in AMD enhances surface activation and electrostatic stabilisation of reactive functional groups, thereby facilitating ligand exchange reactions (Macena et al., 2025). In this process, hydrated Pb²⁺ species displace surface-bound hydroxyl or water molecules to form ≡O–Pb and ≡N–Pb coordination bonds (He et al., 2021). The attainment of near-complete Pb²⁺ removal (>99%) at equilibrium supports the predominance of chemisorption over physical adsorption, consistent with the formation of thermodynamically stable metal–ligand complexes. Collectively, these results demonstrate that the PHA ligand functions as a selective soft-base chelating surface, maintaining high Pb²⁺ affinity even under chemically complex AMD conditions, highlighting its suitability for practical wastewater remediation applications.

3.4 Isotherm model

The Langmuir, Freundlich, and Temkin isotherm models were applied, as shown in Table 2, to evaluate the equilibrium adsorption behaviour of Pb²⁺ in both single-metal and AMD systems. Notably, the experimentally observed adsorption capacity under optimal conditions exceeded the Langmuir-derived q_max, suggesting that Pb²⁺ uptake by the poly (hydroxamic acid) (PHA) ligand was not governed solely by ideal monolayer adsorption. Instead, removal appeared to be driven by coordination-enhanced binding at hydroxamate sites, where oxygen-donor groups interacted with Pb²⁺ through chelate-like Pb–O bonding (Wang et al., 2026).

TABLE 2. Isotherm parameters for Pb²⁺ adsorption in single-metal and acid mine drainage (AMD) systems.

Bold values indicate the parameters associated with the best-fitting adsorption model, as determined by the highest coefficient of determination (R²) and the closest agreement between the experimental (q_e) and calculated adsorption capacities.

While HSAB theory provides a useful interpretive framework, Pb²⁺ selectivity in AMD was more convincingly explained by its high polarizability, relatively high covalent index, and strong tendency to form stable surface complexes under competitive conditions. Accordingly, although the Langmuir model adequately described the equilibrium trend, it likely underestimated the full adsorption potential of the PHA ligand in real AMD systems, where site heterogeneity, protonation–deprotonation effects, and multimetal competition influenced Pb²⁺ binding behaviour (Wei et al., 2025).

The exceptionally high coefficients of determination (R² = 0.9992 and 0.9996, respectively) indicated that the Langmuir isotherm provided an excellent empirical fit to the experimental data for both single Pb²⁺ and AMD systems. However, this should not be interpreted as definitive evidence of a purely homogeneous monolayer adsorption mechanism, but rather as a mathematical representation of the dominant adsorption trend. The Langmuir maximum adsorption capacity (q_max) for single Pb²⁺ was 74.63 mg/g, which closely corresponded to the adsorption plateau observed under optimised experimental conditions. In contrast, a lower q_max value of 67.11 mg/g was obtained for AMD, suggesting that co-existing metal ions competitively occupied high-affinity adsorption sites, thereby reducing the effective Pb²⁺ uptake. According to HSAB theory, Pb²⁺ is classified as a borderline-to-soft Lewis acid and therefore exhibits affinity toward soft and borderline donor atoms, particularly oxygen- and nitrogen-containing functional groups with high polarizability. The relatively high Langmuir affinity constants (K_L = 0.4149 L/mg for single Pb²⁺ and 0.2822 L/mg for AMD) indicated strong Pb²⁺adsorbent interactions, consistent with specific inner-sphere complexation rather than nonspecific electrostatic attraction. The lower K_L value in AMD further reflected competitive adsorption, where other metal ions present in the matrix partially occupied preferential binding sites (Alfarra et al., 2004; Babel and Kurniawan, 2003).

The dimensionless separation factor (R_L) values, ranging between 0.03 and 0.09 for both systems, confirmed favourable adsorption across the investigated concentration range. The slightly lower R_L value obtained for AMD suggested that adsorption remained favourable even at higher initial metal loadings, despite reduced site selectivity due to competitive interactions (Li et al., 2022).

The Freundlich model (Table 2) exhibited lower correlation coefficients (R² = 0.9293 for single Pb²⁺ and 0.8724 for AMD), indicating that it was less suitable for describing the adsorption behaviour. Although the Langmuir model provided the best fit to the equilibrium data, the Freundlich constants (n > 1) further confirmed favourable adsorption and suggested the presence of energetic heterogeneity. The higher n value observed for AMD (n = 5.6625) was attributed to multi-metal competition and differential site occupation governed by HSAB affinities, resulting in a broader distribution of adsorption energies without altering the dominant monolayer adsorption mechanism (Ruiz et al., 2021).

Additionally, the Temkin model (Table 2) yielded a fair match (R² = 0.9877), suggesting that the heat of adsorption decreased with increased surface coverage and that adsorbate–adsorbent interactions influenced the adsorption process (Serafin and Dziejarski, 2023). The comparable Temkin B and b values obtained for single Pb²⁺ and AMD indicated that, although competitive adsorption reduced the overall adsorption capacity, the intrinsic Pb²⁺ surface interaction energy remained essentially unchanged. This observation further supports an HSAB-controlled adsorption mechanism, dominated by specific Pb²⁺ coordination to soft donor sites rather than purely physical adsorption.

3.5 Kinetic models

As presented in Table 3, the pseudo-second-order (PSO) model most accurately described the adsorption kinetics of Pb²⁺ onto the PHA-modified natural adsorbent in both single-solute systems and acid mine drainage (AMD). This is evidenced by an R² value of one and close agreement between the predicted and experimental equilibrium capacities (q_e ≈ 85 mg/g). On the other hand, the pseudo-first-order (PFO) model considerably overestimated q_e and yielded poorer correlation values (R² = 0.7763 for single-solute Pb²⁺ and 0.7276 for Pb²⁺ in AMD). This suggests that Pb²⁺ uptake was not controlled by a first-order physisorption process. The superior PSO fit indicated that adsorption was primarily controlled by surface complexation rather than diffusion-controlled physisorption (Mohamed et al., 2024).

TABLE 3. Kinetic models for Pb²⁺ adsorption under single-solute and Acid Mine Drainage (AMD) Conditions.

Bold values denote the parameters of the best-fitting models used for interpretation of the adsorption mechanism. The Langmuir isotherm provided the best description of the equilibrium data, while the pseudo-second-order (PSO) model gave the best fit to the kinetic data based on the R² values.

Although the apparent kinetic constants decreased in the AMD matrix, reflecting multi-ion competition and elevated ionic strength, the equilibrium capacity remained essentially unchanged. This preservation of q_e demonstrated that Pb²⁺ retained a strong affinity for the hydroxamate functional groups even in the presence of competing metals (Aranda-García et al., 2020). The adsorption mechanism was therefore attributed predominantly to inner-sphere chelation between Pb²⁺ and the bidentate (O, O-coordination) donor sites of the hydroxamate groups on the PHA surface. Similar chelation-driven adsorption mechanisms involving functionalised ligands were reported for Pb²⁺ removal using amidoxime-based materials (Mohery et al., 2025), supporting the role of inner-sphere complexation in chemically modified adsorbents.

In mixed-metal systems, hydroxamate ligands exhibited strong intrinsic affinity for hard Fe³⁺, consistent with established hydroxamate–Fe³⁺ coordination chemistry, and formed stable complexes with Cu²⁺ and Pb²⁺. At the same time, comparatively weaker interactions were generally expected for Zn²⁺ and Mn²⁺. However, under the present AMD conditions (pH 6, elevated sulfate), effective competition was strongly influenced by metal speciation. At circumneutral pH, Fe³⁺ underwent extensive hydrolysis and partial transformation to hydroxide species, potentially reducing the fraction of freely competing ferric ions in solution. Simultaneously, Pb²⁺ formed aqueous sulfate complexes that decreased the activity of free Pb²⁺ and moderated the adsorption rate; however, these species underwent ligand exchange to form stable inner-sphere surface complexes with hydroxamate groups. Similar metal-selective chelation behaviour was reported for hydroxamate ligands, which exhibited strong intrinsic affinity for hard Fe³⁺ through exclusive O, O-coordination and formed high-stability complexes with Cu²⁺ and Pb²⁺, while comparatively weaker interactions were observed for other borderline divalent metals (Bátka and Farkas, 2006).

Under pH six conditions, where Fe³⁺ hydrolysis restricted effective competition, Pb²⁺ functioned as a borderline/soft-leaning Lewis acid capable of forming stable inner-sphere chelates with the oxygen-donor hydroxamate ligands. This coordination compatibility helped explain the observed selective Pb²⁺ retention in the multi-metal AMD matrix. Collectively, these results confirmed that Pb²⁺ removal in AMD was governed by chelation-driven surface complexation, with matrix effects influencing adsorption kinetics but not the equilibrium capacity. The conserved q_e under competitive conditions highlighted the selective affinity of the PHA-modified adsorbent for Pb²⁺, supporting its applicability for targeted Pb²⁺ separation in sulfate-rich AMD systems.

3.6 Competitive metal ion behaviour

Figure 3 displays the adsorption capacity for Fe³⁺, Cu²⁺, Mn²⁺, Zn²⁺, and Pb²⁺ from AMD as a function of increasing starting metal concentration. The adsorption capacity for each of the metal ions under investigation rose with concentration, suggesting improved use of accessible active sites at higher metal loadings and an increased driving force for mass transfer.

Cu²⁺ continuously demonstrated the best removal effectiveness across the concentration range, surpassing 73,5 mg/g at high doses. At intermediate and high concentrations, Fe³⁺ (71,7 mg/g) and Pb²⁺ (74,2 mg/g) demonstrated almost identical removal efficiencies, whereas Zn²⁺ showed moderate removal. Throughout the concentration range under investigation, Mn²⁺ exhibited the lowest percentage clearance, especially at lower starting concentrations. The adsorption capacity values of all metal ions converged at higher concentrations (125 mg/L), indicating increased binding site saturation and decreased selectivity under high loading circumstances.

3.7 Mechanistic interpretation of metal selectivity in multi-metal AMD systems

Table 4 summarises the physicochemical characteristics and HSAB classifications of the examined AMD metal ions to rationalise the observed selectivity trends. These variables offer a framework for analysing the relative adsorption behaviours in terms of donor-acceptor compatibility, ionic radius, and charge density.

TABLE 4. Physicochemical properties and HSAB classification of AMD metal ions.

Hydroxamate functional groups acted as oxygen-donor ligands and facilitated metal binding predominantly through chelation-driven inner-sphere surface complexation. The observed selectivity trends were interpreted in terms of intrinsic metal-ion properties and Hard–Soft Acid–Base (HSAB) principles. Adsorption behaviour in multicomponent systems is strongly governed by charge density, ionic radius, electronic configuration, and donor–acceptor compatibility (Youssif et al., 2024; Kong et al., 2020).

Fe³⁺ exhibited enhanced removal, which was consistent with its high charge density, strong Lewis acidity, and hydrolysis propensity under circumneutral conditions. Hydrolysis likely promoted the formation of surface-associated species in addition to direct coordination with hydroxamate groups. In contrast, Mn²⁺ showed lower uptake than expected, despite being classified as a hard acid. This weaker removal was attributed to its larger ionic radius and reduced tendency to form strong inner-sphere complexes with oxygen-donor ligands.

Among the borderline acids, Cu²⁺ demonstrated superior removal relative to Zn²⁺. Although both ions possess similar ionic radii, Cu²⁺ benefited from a favourable electronic configuration and stronger ligand-field stabilisation, thereby enhancing specific coordination interactions, whereas Zn²⁺ adsorption appeared to rely more on electrostatic interactions (Kong et al., 2020). Pb²⁺, characterised by higher polarizability and borderline acid behaviour, also formed stable surface complexes.

Consistent with HSAB predictions, Cu²⁺ and Pb²⁺ exhibited preferential complexation depending on donor compatibility. Similar selectivity patterns were reported for functionalised mesoporous adsorbents, where donor hardness governed metal affinity (Lee et al., 2016). At elevated metal concentrations, removal efficiencies converged, indicating progressive site saturation and intensified competition, characteristic of capacity-limited behaviour in multi-metal AMD systems.

3.8 Thermodynamics analysis

The thermodynamic parameters of enthalpy change (ΔH°), entropy change (ΔS°), and Gibbs free energy change (ΔG°) (Table 5) were used to assess the adsorption of Pb²⁺ from aqueous solution and (Table 6) Fe³⁺, Cu²⁺, Mn²⁺, Zn²⁺, and Pb²⁺ ions from acid mine drainage (AMD) onto the PHA ligand.

TABLE 5. Thermodynamic parameters of metal ions adsorption onto PHA ligand.

TABLE 6. Thermodynamic parameters of metal ions adsorption onto PHA ligand.

Overall, of the thermodynamic, kinetic, and equilibrium results point to a chemisorption-driven coordination mechanism that regulates Pb²⁺ adsorption onto the PHA ligand. This process is affected by temperature, active site availability, and solution composition. Adsorption was endothermic and spontaneous, especially at high temperatures, as indicated by the thermodynamic parameters listed in Tables 5, 6, which showed positive ΔH° and negative ΔG° values. These patterns were corroborated by the observed increase in adsorption capacity with temperature, suggesting improved ion mobility and activation of coordination sites within the polymer matrix (Tumampos et al., 2021).

Although the pseudo-second-order model most adequately characterised Pb²⁺ uptake (Section 3.5), the kinetic behaviour validated the thermodynamic interpretation. This suggested that surface complexation reactions, not diffusion-controlled physical adsorption, were the rate-limiting step. The thermodynamic evidence for entropy-driven adsorption and bond formation between Pb²⁺ ions and hydroxamate donor groups supported this behaviour (Mohapatra et al., 2009). Additionally, equilibrium data supported the thermodynamic results. The experimental data (Section 3.4) were best fit by the Langmuir model, indicating monolayer adsorption on a limited number of energetically homogeneous, high-affinity sites. The significant affinity suggested by the Langmuir parameters indicated specialised inner-sphere Pb²⁺ coordination to hydroxamate functional groups and was consistent with the spontaneous ΔG° values shown in Tables 5, 6. The principal adsorption mechanism in the AMD system remained unaltered even though competing ions marginally decreased apparent capacity and adsorption rate. This suggests that matrix factors largely controlled site accessibility rather than intrinsic binding strength (Yin et al., 2024). Overall, a single adsorption mechanism, regulated by ligand-controlled inner-sphere chelation, was consistently confirmed by the thermodynamic data (Tables 5, 6), as well as by the kinetic (Section 3.5) and isotherm (Section 3.4) studies.

3.9 Comparative evaluation of Pb²⁺ adsorption performance and mechanistic implications of poly (hydroxamic acid) (PHA)

Table 7 situates the Pb²⁺ adsorption performance of the poly (hydroxamic acid) (PHA) adsorbent relative to a range of conventional and advanced materials. Overall, PHA demonstrated competitive adsorption capacity, outperforming low-cost adsorbents such as unmodified biochar and activated bentonite, and falling within the upper range reported for modified mineral-based materials such as zeolites. While higher capacities have been reported for graphene oxide and activated carbon, these materials are typically evaluated under idealised single-metal conditions and rely predominantly on surface area-driven adsorption.

TABLE 7. Comparison of maximum adsorption capacities (q_max) for Pb²⁺ removal using various adsorbents under different experimental conditions.

In contrast, the PHA adsorbent maintained relatively high Pb²⁺ uptake under mild pH (≈6) and ambient temperature (299 K), conditions that are representative of real AMD systems. Its performance in multi-metal environments further highlights the role of selective, functional group–mediated interactions rather than nonspecific adsorption.

Collectively, these results underscore the practical advantage of PHA as a selective adsorbent for Pb²⁺ removal in complex aqueous matrices, where adsorption behaviour is governed by coordination interactions and competitive effects.

3.10 Desorption and adsorbent reuse

The primary objectives of the desorption studies were to determine whether the spent adsorbent was recyclable after several cycles of adsorption and desorption, and whether it would be economically and practically feasible for use in water treatment. To determine the best eluent for releasing bound metal ions from the adsorbent surface, a crucial step for process optimisation, regeneration effectiveness, and long-term operational sustainability, desorption tests were also carried out. These investigations also helped distinguish between chemically coordinated binding and physical adsorption by shedding light on the strength and reversibility of adsorbate–adsorbent interactions. Desorption analysis also helped determine the adsorbent’s stability, resistance to structural deterioration, and potential for metal recovery, all crucial factors for the practical application of adsorption technology. Equation 9 was used to get the percentage desorption efficiency of heavy metal ions:

Desorption % = amount desorbed amount adsorbed × 100

Figure 4 illustrates how the eluent utilised had a significant impact on desorption efficiency. Across the three cycles, mineral acid consistently produced the best regeneration performance, followed by water, weak acid, and alkaline solution. This trend confirmed that the adsorbed metals were mainly bound by reversible coordination rather than by weak physical forces, indicating that proton-assisted ligand displacement was the predominant regeneration mechanism (Bayuo et al., 2024). Partial site saturation, slight structural changes to functional groups, or the formation of tightly bound remnant complexes were suggested by the gradual decrease in desorption efficiency over the three regeneration cycles. However, a significant amount of regeneration capacity was maintained, indicating that the PHA ligand had good operational stability and recyclability for repeated use in metal removal systems.

The reusability results showed that, despite slow reductions, the PHA ligand maintained a significant level of adsorption performance across five adsorption–desorption cycles. For all metal ions, the removal efficiency gradually declined from Run one to Run 5, as seen in Figure 5a, suggesting either partial desorption or active-site loss during regeneration. Similarly, the adsorption capacity (q_e) decreased with each cycle in Figure 5b. Pore obstruction, slight structural fatigue of functional groups, or the creation of firmly bound residual complexes that the eluent was unable to remove completely were probably the causes of this drop. While Mn²⁺, Zn²⁺, and Pb²⁺ showed more pronounced declines, Fe³⁺ and Cu²⁺ maintained greater stability throughout the five cycles, indicating stronger coordination stability and superior regeneration resistance (Crini and Lichtfouse, 2019). Excellent structural stability and appropriate regeneration potential for repeated metal ion removal applications were confirmed by the ligand’s capacity to sustain significant removal efficiency after five cycles, despite the gradual loss.

4 Conclusion

Using a poly (hydroxamic acid) ligand, this study examined the selective adsorption of Pb²⁺ from synthetic solutions and from actual acid mine drainage (AMD). At pH 6, the adsorbent’s highest capacity was 86 mg/g. The Langmuir isotherm best explained the equilibrium data, whereas the pseudo-second-order model described the kinetic behaviour. Thermodynamic data demonstrate that adsorption is endothermic and spontaneous. Notably, the experimental adsorption capacity exceeded the Langmuir-derived q_max, indicating that Pb²⁺ uptake is not governed solely by ideal monolayer adsorption but by coordination-enhanced mechanisms.

Adsorption was primarily driven by inner-sphere complexation at hydroxamate functional groups, where oxygen-donor atoms form stable Pb–O interactions. While HSAB theory provides a useful framework, Pb²⁺ selectivity is more accurately attributed to its high polarizability and strong tendency to form stable surface complexes under competitive conditions. The higher experimental capacity relative to q_max reflects the heterogeneous and multidentate nature of the PHA surface, which is not fully captured by classical isotherm models. In multi-metal AMD systems, the adsorbent maintained a high Pb²⁺ removal efficiency, demonstrating its selectivity in competitive settings. Additionally, the material showed remarkable reusability and operational stability, maintaining over 80% removal effectiveness after five adsorption–desorption cycles.

Based on a sustainability standpoint, the use of mineral acid as an efficient desorbing agent implies that Pb²⁺ can be recovered in a concentrated form, offering the possibility of resource recovery and metal valorization. However, partial site saturation and potential structural alterations are indicated by the slow decrease in adsorption capacity across repeated cycles, which may restrict long-term reuse. In these situations, the spent adsorbent might be used again in secondary applications that call for a lesser adsorption capacity. As an alternative, the Pb-loaded adsorbent should be stabilized before disposal to avoid possible metal leaching and guarantee environmental safety. When evaluating the PHA system’s long-term environmental and practical usefulness in acid mine drainage treatment, these factors are crucial.

Overall, these results show that hydroxamate-functionalized materials are efficient, affordable, and sustainable adsorbents for the selective removal of Pb2+ from complex mining effluents. They also emphasize how crucial it is to take adsorption mechanisms and end-of-life management into account when designing workable water treatment systems. While this study focused on batch adsorption, fixed-bed column studies are required to evaluate performance under continuous flow conditions. Future work will therefore investigate breakthrough behaviour and column performance to assess scale-up potential.

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Footnotes

Edited by: Gopal Krishan, National Institute of Hydrology (Roorkee), India

Reviewed by: Muhammad Faheem, China University of Geosciences Wuhan, China

Khaled Al-zawahreh, Hashemite University, Jordan